The present invention relates to a plasma processing apparatus and, more particularly, to a plasma processing apparatus for forming thin films by introducing a microwave into a plasma chamber through a dielectric window with an external magnetic field being applied to the plasma chamber, converting a raw material in the plasma chamber into plasma by ECR (Electron Cyclotron Resonance), and radiating the resultant plasma onto a specimen.
FIG. 14 shows an example of a conventional plasma processing apparatus. As a plasma processing apparatus of this type, "Plasma Deposition Apparatus" disclosed in Japanese Patent Publication No. 62-43335 (Japanese Patent Application No. 55-57877) and "Plasma Processing Method and Apparatus" disclosed in Japanese Patent Laid-Open No. 1-97399 (Japanese Patent Application No. 63-98330) are known.
Referring to FIG. 14, reference numeral 10 denotes a specimen chamber; 20, a plasma chamber; and 30, a microwave supplying means. The specimen chamber 10 has a specimen holder 11 for holding a specimen 40 and communicates with an exhaust passage 13 through a ventilation hole 12 formed below the specimen holder 11. The specimen chamber 10 also communicates with the plasma chamber 20 through a plasma extracting opening 21 on the side away from the exhaust passage 13.
A first gas is introduced into the plasma chamber 20 from a first gas source GS1 through a gas inlet pipe 22 as a first gas introducing system. An annular pipe 23 with a plurality of small holes is arranged outside (in this apparatus, inside the specimen chamber 10) and close to the plasma extracting opening 21, introducing a second gas to the specimen chamber 10 from a second gas source GS2 through a gas inlet pipe 24 as a second gas introducing system. An annular cooling unit 25 is also arranged around the plasma chamber 20 to supply a coolant, such as water, from a coolant source CS through a cooling pipe 26.
In the plasma generating chamber 20, a microwave introducing window (dielectric window) 27 consisting of, e.g., a quartz glass plate, is provided on the end face (the upper surface in FIG. 14) opposing the opening 21. A microwave from the microwave supplying means 30 is introduced into the plasma chamber 20 through this microwave introducing window 27 while maintaining the vacuum degree. A microwave mode transducer 35 is arranged between a rectangular waveguide 33 and the microwave introducing window 27 to tune the rectangular waveguide mode of a microwave with the microwave propagation mode in plasma.
An annular magnetic coil 50 is arranged around the outer circumferential surfaces of the plasma chamber 20 and the microwave mode transducer 35 and a partial outer circumferential surface of the rectangular waveguide 33 connected to the converter 35. The magnetic coil 50 generates an external magnetic field B required to generate ECR (875 G at a microwave frequency of 2.45 GHz). In practice, the magnetic coil 50 is also cooled like the plasma chamber 20.
In the apparatus with the above arrangement, the first gas introduced as a raw material into the plasma chamber 20 is excited under the ECR condition by a microwave introduced through the microwave introducing window 27 and is converted into plasma. The plasma thus generated is guided to a position above the specimen holder 11 in the specimen chamber 10 by using a magnetic field gradient, forming a thin film on the specimen 40 placed on the specimen holder 11.
The plasma processing apparatus with the above arrangement which forms thin films by using ECR plasma has various characteristics, such as a low gas pressure (10.sup.-4 to 10.sup.-5 Torr), a high activity, and a low damage, and can therefore form dense, high-quality thin films of, e.g., SiO.sub.2, Si.sub.3 N.sub.4, and SiC at low temperatures without heating.
In the arrangement shown in FIG. 14, however, the microwave introducing window 27 is in contact with the plasma chamber 20, and so a thin film is also deposited on the microwave introducing window 27 during formation of the thin film. In forming a conductive film, therefore, the conductive film is also formed on the microwave introducing window 27 with the result that a microwave is reflected or absorbed by that film. Consequently, plasma can no longer be maintained to make the film formation impossible. In some cases, the microwave introduction conditions gradually change to impair the reproducibility. As described above, the conventional apparatus with the above arrangement cannot form conductive films stably for long periods of time because the conductive films are deposited on the microwave introducing window 27.
ECR plasma apparatuses as shown in FIGS. 15, 16, and 17, which aim to avoid the deposition of conductive films on the microwave introducing window 27 described above, are also known to those skilled in the art. However, these apparatuses have their respective problems. In FIGS. 15, 16, and 17, the same reference numerals as in FIG. 14 denote the same parts or parts having the same functions.
First, in the arrangement shown in FIG. 15, the microwave introducing window 27 is arranged at a position at which the microwave introducing window 27 cannot be seen directly from a microwave introducing opening 28, i.e., a position at a dead angle from the microwave introducing opening 28. In addition, a ferromagnetic member 71, such as a yoke, is arranged between the magnetic coil 50 and a vacuum waveguide 72. A microwave is introduced into the plasma chamber 20 through the microwave introducing window 27 and the vacuum waveguide 72. The other arrangement is the same as that shown in FIG. 14.
In the arrangement shown in FIG. 15, since the microwave introducing window 27 is arranged at a position to which no plasma particles fly directly, conductive films are not deposited easily on the microwave introducing window 27 compared to the arrangement shown in FIG. 14. In this case, the generation of plasma inside the vacuum waveguide 72 is a problem. In this arrangement, however, the generation of plasma inside the vacuum waveguide 72 is suppressed by decreasing the magnetic flux density inside the vacuum waveguide 72 by arranging the ferromagnetic member 71 around the vacuum waveguide 72 (Jpn. J. Appl. Phys. 28 (1989) L503-L506 "A Few Techniques for Preparing Conductive Material Films for Sputtering-Type Electron Cyclotron Resonance Microwave Plasma" by Morito Matsuoka and Ken'ichi Ono).
In this arrangement, however, since the magnetic field in the vacuum waveguide 72 is weak, a microwave is cut off in the vacuum waveguide 72 by plasma diffusing from the plasma chamber 20. This causes reflection of the microwave to result in a low plasma density.
In the apparatus shown in FIG. 16, like the above arrangement, the microwave introducing window 27 is arranged at a position to which no plasma particles fly directly, and consequently conductive films are not easily deposited on the microwave introducing window 27. The vacuum waveguide 72 is coupled to the plasma chamber 20 through a portion between two magnetic coils 51 and 52. A microwave from a microwave source 31 passes through the microwave introducing window 27, propagates in the vacuum waveguide 72 in a direction perpendicular to an external magnetic field (in a direction indicated by an arrow B in FIG. 16), and is introduced into the plasma chamber 20. In this arrangement, the generation of plasma inside the vacuum waveguide 72 is a problem. However, the plasma generation inside the vacuum waveguide 72 can be prevented because the vacuum waveguide 72 is coupled to the plasma chamber 20 such that the propagating direction of a microwave is perpendicular to the external magnetic field and the microwave electric field is parallel to the external magnetic field. In addition, the diffusion of plasma into the vacuum waveguide 72 is also prevented by using the property of plasma that it is captured by lines of magnetic force and therefore does not diffuse easily in a direction perpendicular to the lines of magnetic force (Jpn. J. Appl. Phys. 28 (1989) L503-L506 "A Few Techniques for Preparing Conductive Material Films for Sputtering-Type Electron Cyclotron Resonance Microwave Plasma" by Morito Matsuoka and Ken'ichi Ono).
In this arrangement, however, a microwave propagates as an orthogonal wave (one of propagation modes of a microwave in plasma, in which the propagating direction of a microwave is perpendicular to an external magnetic field and the direction of an electric field is parallel to the external magnetic field) in the plasma chamber 20. Therefore, a cut-off phenomenon of a microwave cannot be avoided, and this causes reflection of a microwave to lead to a low plasma density.
That is, plasma can be generated easily when (a) an appropriate vacuum is realized, (b) an external magnetic field is present in a direction perpendicular to a microwave electric field, and (c) a microwave electric field is present. Plasma becomes difficult to generate when at least one of these conditions is not established. To stably generate high-density ECR plasma, on the other hand, it is necessary to satisfy the following three conditions at the same time: (d) a microwave is introduced into a plasma chamber along an external magnetic field from the side having a magnetic field higher than the ECR condition in order to prevent cut-off of the microwave; (e) the direction of a microwave electric field is perpendicular to that of the external magnetic field; and (f) an appropriate vacuum is realized.
In the vacuum waveguide, a proper vacuum is maintained and a microwave electric field exists. To prevent the generation of plasma in the vacuum waveguide, therefore, the microwave electric field may be set to be parallel to the external magnetic field, or the external magnetic field may be set to 0.
In the arrangement shown in FIG. 15, the generation of plasma inside the vacuum waveguide 72 is avoided by weakening the magnetic field inside the vacuum waveguide 72. If, however, plasma diffuses into the waveguide 72, the cut-off phenomenon of a microwave occurs because of the weak magnetic field, resulting in a low plasma density. In the arrangement shown in FIG. 16, on the other hand, the generation of plasma is suppressed by coupling the vacuum waveguide 72 to the side surface of the plasma chamber 20 so that the microwave electric field is parallel to the external magnetic field. However, the cut-off phenomenon of a microwave takes place in the plasma chamber 20 to make it difficult to increase the plasma density.
In the arrangement shown in FIG. 17, an RF power source 81 is used to supply radio-frequency power (RF power) to the microwave introducing window 27. That is, Ar gas is introduced into the plasma chamber 20 and converted into plasma, and the RF power is supplied from the RF power source 81 to the microwave introducing window 27. This generates a self-bias, and Ar ions in the plasma are accelerated and collided against the microwave introducing window 27 by the self-bias generated. The consequent sputtering effect of the Ar ions prevents deposition of conductive films on the microwave introducing window 27 (Preparation of TiN films by Electron Cyclotron Resonance Plasma Chemical Vapor Deposition. Takashi Akahori, Akira Tanihara and Masashi Tano: Japanese Journal Of Applied Physics Vol. 30, No. 12B, December, 1991, pp. 3558-3561).
In this arrangement, however, the collision of Ar ions poses a problem of damage to the microwave introducing window 27, and this results in a problem of mixing of impurity materials into films to be formed. In addition, it is necessary to additionally provide a cooling mechanism for preventing the temperature rise of the microwave introducing window 27 and a means for supplying an RF power to the microwave introducing window 27.
An arrangement in which a microwave is introduced obliquely to the central axis of lines of magnetic force by using one or more waveguides in order that no plasma particles fly directly to the microwave introducing window is also known to those skilled in the art. As an example, Japanese Patent Laid-Open No. 61-281883 has proposed a similar arrangement although the purpose of the arrangement is different. In such an arrangement, however, an electric field component perpendicular to a magnetic field is unavoidably generated regardless of whether the waveguide is of a TE mode or a TM mode. In addition, the above literature does not mention the magnetic field strength at the position of the microwave introducing window. However, in attempting to form a magnetic field meeting the ECR condition inside the plasma chamber in the arrangement disclosed in the above literature, the magnetic field strength at the position of the microwave introducing window becomes lower than the ECR condition. As a result, the ECR condition is formed in a vacuum portion of the waveguide between the microwave introducing window and the plasma chamber. In this arrangement, therefore, plasma is undesirably generated inside the waveguide, and consequently a microwave becomes difficult to introduce to the ECR point inside the plasma chamber. This makes efficient plasma generation impossible.